travel antigen test in khobar/dammam

Sorry for replying late, but this could be helpful for future guests.

I think you mean "PCR test" for (COVID-19). Most hospitals offer it here. You can contact the nearest hospital and ask about their prices as prices differ. Also, most of the time it depends on how fast you need your results so for example results in 12 hours will be more expensive than 24 hours.
Also, there are drive-through PCR testing centers such as "Hyatt Lab Drive Through PCR testing centre" type that in Google Maps and you will find it.

Again, sorry if the info is not helpful because it's late

Antigen is what you would call the rapid test.   And generally speaking, it is not accepted for travel abroad.   Since it is a rapid test, no one gives a certificate with a QR code or even stamp, in Saudi.  Only the pharmacies in Bahrain give a stamped certificate (that was for the causeway when there was a requirement for the same) with the test itself attached.

You need to do a primary PCR and then in some destinations, rapid antigen on arrival.    But again, it depends on which destination you are going to.